1. Field of the Invention
The present invention relates to a low-viscosity ultraviolet-curable silicone composition for a release paper.
2. Description of the Prior Art
A variety of ultraviolet-curable silicone compositions have been proposed. In recent years, a multitude of curing methods have been proposed in which an epoxy group-containing silicone is cured using an onium salt photoinitiator as a catalyst (see Patent Documents 1 to 3). This type of curing method differs from curing methods that utilize a radical reaction, and because the curing reaction proceeds favorably in air with no curing inhibition caused by oxygen, compositions that use this curing method are now being used for treatment agents for the back surface of pressure-sensitive adhesive tapes, and as coating materials for release papers used with pressure-sensitive adhesive labels. Furthermore, from the viewpoints of health and safety, there is strong demand for ultraviolet-curable silicone compositions that do not require dilution with a solvent, but can be applied directly to a substrate in a solventless state, and subsequently cured.
Generally, in those cases where a solventless silicone is applied, provided the substrate surface is flat, even a high-viscosity solventless silicone can be used. However, when a solventless silicone is coated onto a substrate surface that lacks planarity and has significant unevenness, if the viscosity of the silicone is too high, then the quantity of silicone applied must be increased to cover the unevenness on the substrate surface. Accordingly, in those cases where a solventless silicone is coated onto a substrate having significant unevenness, the viscosity of the silicone must be lowered.
Furthermore, in recent years, for reasons of health and safety, hot-melt pressure-sensitive adhesives that use no solvent have become widely used as the pressure-sensitive adhesives for pressure-sensitive adhesive tapes and pressure-sensitive adhesive labels. Hot-melt pressure-sensitive adhesives generally tend to be harder than pressure-sensitive adhesives prepared using solvent dilution, and therefore when a release paper that has been bonded to a hot-melt pressure-sensitive adhesive in a releasable manner is peeled away from the adhesive, a peeling noise tends to occur. This peeling noise tends to be smaller when the peel strength required to peel the release paper from the pressure-sensitive adhesive has a positive dependency on the peel speed (in other words, when the peel strength increases as the peel speed is increased). Accordingly, there is a demand for silicone compositions for release papers in which the peel strength exhibits a positive dependency relative to the peel speed.
[Patent Document 1] GB 2123842 A
[Patent Document 2] EP 0391162 A2
[Patent Document 3] US 2003/0232900 A1